MHD equilibria with magnetic islands in the TJ-II heliac

Abstract

ABSTRACTMagnetic islands normally form at the low-order magnetic rational surfaces in toroidal confinement systems when a symmetry-breaking perturbation arises. In stellarators, the equilibrium state for the plasma once the perturbation has created the islands is not easy to describe due to its complexity. Numerical computation of equilibria can be done following a variational principle that minimizes the energy under given boundary constraints when no islands are allowed, as the VMEC code does. Islands can be included by adding helical perturbations and allowing magnetic field line slipping, which was incorporated in the SIESTA code. Here we use these two codes for the TJ-II heliac in order to compute the MHD equilibrium state that provides the environment for the occurrence of all other plasma processes. The computation was done in two parts, first, obtain an equilibrium with VMEC for the whole toroidal cycle and second, superimpose the magnetic islands to obtain a final equilibrium with SIESTA by adjusting the appropriate parameters.